Telephone, cable television and other communication and control cables are often buried or placed in underground duct structures. For this type of cable placement, the most significant cause of cable outages is from dig ups by contractors. In an effort to minimize inadvertent dig ups, “call before you dig” programs are heavily promoted. The operating company must then be able to quickly and accurately locate and mark the buried cable.
Methods have been developed and are in commercial use which place a locating tone or signal in the form of an oscillating current at a predetermined frequency on the cable armour or shield. A special receiver with magnetic field detecting coils is used to sense the tone current travelling along the cable. The strength of the received signal is directly proportional to the magnitude of the tone current in the cable sheath directly below the receiver.
The transmission circuit for the tone current is formed by the metal armour or shield and insulated by the plastic cable jacket from earth which forms the return conductor. The circuit is basically a coaxial transmission path with the insulated cable armour forming the inner conductor and the surrounding earth forming the outer conductor.
The tone current must be present on all segments of a cable at a level greater than the minimum current dictated by the receiver sensitivity. This requires a termination at the end of the cable to draw at least the minimum amount of current. A distribution cable typically has a number of branch cables which must also draw enough tone current for cable locating. Since the current is heavily attenuated by the cable, the terminations near the source will draw a much higher current level than the distant terminations. To compensate for this, most installations use terminators with different signal load impedances for near, middle and far terminations. In addition to the inconvenience of using different impedances for different terminations, the known systems require recalculation and replacement of the terminators when an additional branch is connected.
Where a cable is damaged, the current level may fall below the minimum, making it difficult or impossible to locate the damaged cable.